Patterns of body size with depth were evaluated for marine isopods crustaceans in the Nordic Seas (the Iceland Sea, the Norwegian Sea), within the depth range of 18 to 3900 m. In all, 55 isopod species were considered, of which most species (85%) belong to the suborder Asellota. The pattern of body size with depth differed considerably depending upon groups of isopods considered. When all isopod species were included in the analysis no pattern in average maximum body size (calculation based on all species at each locality) was observed with depth. There was, however, a significant increase in the average maximum size of asellote isopods with increasing depth and also when munnopsid asellotes were considered only. There was further a significant correlation between the average maximum body size of both asellote isopods and munnopsid asellotes and the nearbottom temperature, while no pattern was observed when all isopods were considered. No correlation was found between the average maximum body size and the species richness of the isopods. There was, however, a highly significant correlation between the variation of the maximum size of the asellote isopods and the species richness. It is concluded that size-depth related patterns are more likely to occur within a homogenous group than a heterogeneous one as functional aspects may be more size determining than physiological factors, such as the temperature. We suggest that the temperature, in connection with food availability, constraints size-depth related patterns of asellote isopods in the Nordic Seas. The pattern of diversity of the deep Arctic Ocean and Nordic Seas may be reflected in large size distribution of asellotes at high diversity sites but narrower size range in low diversity areas.